Experimental study of a free-surface circular liquid sheet using planar laser-induced fluorescence

Abstract

This work presents an experimental study of various spatiotemporal events leading to the primary atomization of a radially expanding circular liquid sheet. A cylindrical liquid (water) jet orthogonally made to impinge on the horizontally placed cone-disc deflector to produce a thin free-surface circular liquid sheet from the peripheral edge of the deflector disc. High-speed planar laser-induced fluorescence (PLIF) at 6 kHz framing rate was carried out to capture the atomizing liquid sheet in the side and front views at various jet Weber numbers (${\text{We}}_{{{\text{jet}}}}$) over a range of 994 < ${\text{We}}_{{{\text{jet}}}}$ < 8029. The side view PLIF comprising of three fields of view to capture the complete atomization process of expanding liquid sheet in a time-resolved manner starting from capturing radial wave features on liquid sheet down to the sheet breakup, ligament formation, and droplet production. The azimuthal wave features were captured in the front view PLIF experiments. The quantitative information of different spatiotemporal features was extracted from experimental data using rigorous image processing methods. Advanced feature extraction and transient analysis methods like proper orthogonal decomposition (POD) was applied to obtain the dominant flow features associated with sheet dynamics. The POD analysis showed that the most dominant phenomenon in the flow field for ${\text{We}}_{{{\text{jet}}}}$ < 1500 is found to be the oscillation of the liquid sheet, the radial wave propagation in the radial direction for 1500 ≤ ${\text{We}}_{{{\text{jet}}}}$ < 2000, and sheet breaks up at the outer edge for ${\text{We}}_{{{\text{jet}}}}$ ≥ 2000. It is shown that the dominancy of each of these phenomena changes with ${\text{We}}_{{{\text{jet}}}}$. Statistical analysis-based ligament length and thickness ($L_{{{\text{lig}}}}$ and $t_{{{\text{lig}}}}$) follow ${\text{We}}_{{{\text{jet}}}}^{ - 0.6}$ and droplet diameter ($D_{32}$ and $D_{10}$) follow ${\text{We}}_{{{\text{jet}}}}^{ - 0.33}$ power law. Further, the direct correlations obtained showed that the droplet diameter is a stronger function of ligament thickness compared to ligament length.

Graphic abstract

Schematic of optics, laser light and camera arrangements for side view PLIF experiments of a circular liquid sheet. Three FOVs adopted for side view PLIF experiments at different radial locations; R_A= 280 mm for A = 100 mm × 100 mm, R_B = 360 mm for B = 80 mm × 80 mm, and R_C = 420 mm for C = 40 mm × 40 mm

Direct imaging of azimuthal waves of liquid sheet using front view PLIF experimental technique